The present invention relates to a hierarchical transmission digital demodulator for demodulating digital modulated waves multiplexed on a time axis by a plurality of modulation methods in which necessary C/N (ratio of carrier power to noise power) values are different.
A hierarchical transmission method is known by which digital modulated waves multiplexed on a time axis by a plurality of modulation methods in which necessary C/N values, e.g., 8PSK modulation waves, QPSK modulation waves and BPSK modulation waves, are combined to form frames which are repetitively transmitted. With such a hierarchical transmission method, since BPSK modulation waves (including burst symbol signals) provide a wide lock-in range and are easy to capture sync, the sync capture is obtained by receiving the BPSK modulation waves (including burst symbol signals) and after the sync capture is obtained, sequentially input BPSK modulation signals, burst symbol signals (BPSK modulation signals), QPSK modulation signals and 8PSK modulation signals are demodulated (continuously demodulated) in the order of input signals.
However, if the reception C/N value is degraded during the continuous demodulation, the reception state of 8PSK modulation waves requiring a high necessary C/N value is degraded. Therefore, at the receivable limit C/N value of lower hierarchical QPSK or BPSK modulation waves, a carrier slip occurs in the section of 8PSK modulation waves and the frame sync of the system cannot be captured. The necessary C/N value is therefore substantially raised and the reception state becomes unstable.
It is an object of the present invention to provide a hierarchical transmission digital demodulator capable of stable sync capture and stable demodulation by setting a demodulation operation in accordance with the reception C/N value.
A hierarchical transmission demodulator of this invention comprises: first carrier reproducing means for reproducing a carrier in accordance with demodulation outputs that a modulated wave in a header section and a modulated wave of a burst symbol signal are demodulated means for measuring a reception C/N value; second carrier reproducing means for reproducing the carrier in accordance with continuous demodulation outputs if the measured reception C/N value after the sync capture is equal to or larger than a first predetermined threshold; third carrier reproducing means for reproducing the carrier in accordance with demodulation outputs for a hierarchical level except a high hierarchical level if the measured reception C/N value which is equal to or larger than a second threshold which is lower than the first threshold.
According to the hierarchical transmission demodulator of this invention, during the period until sync is captured, the carrier is reproduced by the first carrier reproducing means in accordance with demodulation outputs that a modulated wave in a header section and a modulated wave of a burst symbol signal are demodulated. Accordingly, the carrier can be reproduced reliably. A reception C/N value is measured by the C/N measuring means and if the reception C/N value after the sync capture is equal to or larger than the first predetermined threshold after the sync capture, the carrier is reproduced by the second carrier reproducing means in accordance with the continuous demodulation output. It is there fore possible to prevent jitters, etc. to be caused by inability of following a frequency change while the carrier is not reproduced. If the measured reception C/N value after the sync capture is equal to or larger than the second threshold which is lower than the first threshold, the carrier is reproduced by the third carrier reproducing means in accordance with demodulation outputs for a hierarchical level except a high hierarchical level.
The hierarchical transmission digital demodulator of this invention further comprises reproduction loop characteristic switching means for switching a carrier reproduction to a different carrier reproduction characteristic during carrier reproduction by the first carrier reproducing means and during carrier reproduction by the carrier reproducing means different from the first carrier reproducing means.
According to the hierarchical transmission digital demodulator of this invention, the reproduction loop characteristic is switched between different carrier reproduction characteristics during carrier reproduction by the first carrier reproducing means and during carrier reproduction by the carrier reproducing means different from the first carrier reproducing means.